The present invention, in some embodiments thereof, relates to optical fiber sensing and, more particularly, but not exclusively, to a method and system for sensing properties along an optical fiber.
It is often desirable to monitor strain in a structure, such as a bridge or highway overpass, a building, or a component in a vehicle, in order to get advance warning of fracture or other failure of the structure. However, it is not always convenient (or even necessary) to monitor the strain as it is occurring, and it is sufficient to know merely the maximum strain the structure has experienced in a given timeframe. For example, for vehicles such as aircraft, weight, space and other limitations would preclude monitoring strain of components therein in real time, but for purposes of evaluating the likelihood of future failure, it would be sufficient to know the maximum strain the component in the vehicle experienced during a particular trip. Therefore, it would be convenient to have a means of sensing and maintaining for later measurement the maximum strain the component experienced.
Optical fiber sensors for measuring strain are known in the art, and have many advantages over other types of sensors. They are economical, durable, light-weight, and can be used in electromagnetically noisy environments, and are therefore ideal for use in aircraft. Heretofore, Rayleigh scattering has been used for monitoring the mechanical properties along the fiber in configuration known as optical time-domain reflectometry (OTDR) and its' various implementations [Koyamada, et al., Lightwave Tech. 27, 1142 (2009); Pan et al., in Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2014), paper Th2A.21; A. Champavere, in Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2014), paper W3D.1].
Another type of OTDR is known as Brillouin OTDR technique, which involves measurements based on Brillouin scattering. In an optical fiber, Brillouin scattering is an inelastic phenomenon that results from the interaction of incident optical photons (of an incident optical signal) with acoustic phonons in the medium (the optical fiber). This interaction induces a counter-propagating optical wave (reflected or backscattered optical signal) having a frequency (known as the Brillouin frequency) that is shifted from the frequency of the original incident optical wave. Brillouin scattering in an optical fiber is sensitive to both temperature and strain changes in the optical fiber.
European Publication No. EP0348235 discloses a method for evaluating properties of an optical fiber using Brillouin amplification. The technique uses a non-linear interaction between a first modulated signal light from a first light source and a second signal light from a second light source which counter propagate in an optical fiber, and analyzes the signal waveform of the second signal light which is influenced by Brillouin light amplification.
Additional background art includes Beugnot et al., “Distributed Brillouin sensing with submeter spatial resolution: modeling and processing”, Optics Express, 19, 7381 (2011); Zhao et al., “On-line monitoring system of 110 kV submarine cable based on BOTDR”, Sensors and Actuators A: Physical, 216, 28 (2014); and Minardo et al., “High-Spatial Resolution DPP-BOTDA by Real-Time Balanced Detection”, IEEE Photonics Technology Letters, 26, 1251 (2014).